Traveling wave tube amplifier



Jan. 12, 1960 PING K. TIEN TRAVELING WAVE TUBE AMPLIFIER Filed Deo. 6, 1954 STOP BAND F/G. 3A

/A/l/ENTOR By P. K. 77E/V Z,9Zl,224 .Patented Jan. 12, i960 TRAVELING WAVE TUBE AlVIPLIIFIER Ping K. Tien, Chatham Township, Morris County, NJ., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application December 6, 1954, Serial No. 473,287

18 Claims. (Cl. S15-3.6)

This invention relates to an amplifier of the traveling wave tube type.

A principal object of the present invention is to provide a high gain amplifier for microwave signal energy.

In a traveling wave tube, an electron beam is projected in coupling proximity with an electromagnetic wave propagating along a slow wave interaction circuit. Kinetic energy from the movingelectrons in the beam is transferred to the electromagnetic wave, the amount of energy transferred to the wave being a function of the impedance of the slow wave interaction circuit. For maximum coupling, and therefore maximum gain to the wave, it is important to employ a slow wave circuit having a high impedance. However, it is characteristic that some circuits which have a high impedance for the fundamental component of a traveling wave also have a relatively high impedance for spatial harmonic components of a wave traveling therealong, particularly spatial harmonic components of the backward type. Moreover, it is also characteristic that a slow wave circuit which has a high impedance for spatial harmonic components of the backward type will normally oscillate in a backward wave mode when the beam current is increased beyond a certain point. This can be explained by the fact that operation in a backward wave mode is then regenerative; the electron beam provides positive feedback from regions of high energy level to regions of low energy level along the circuit. Because of this, with such circuits it has not been possible hitherto to use high beam currents and, consequently, high gain and high power leveis could not be obtained. The principles of backward wave oscillations are more fully discussed in an article entitled "Analysis of the Backward-Wave Traveling Wave Tube by I-l. l-leffner which appears in the Proceedings of the Institute of Radio Engineers, vol. 42, No. 6, June 1954.

An important example ot' a slow wave interaction circuit which exhibits these characteristics, and therefore becomes unstable during high-gain operation or at highpower levels, is the bifilar helix. The term "bifilar helix as used herein shall designate a structure composed of two interwound helices, the two helices having substantially equal diameters and pitches. 1t has been found heretofore that a bifilar helix slow wave structure has a higher impedance than an ordinary single wire helix and amplifiers utilizing this type of slow wave structure have been proposed for use in forward wave amplifiers. Moreover, such a circuit has the advantage that it readily adapts itself to electrostatic focusing. A discussion of the embodiment of this type of structure in a traveling wave tube amplifier can be found in United States Patents 2,801,361, issued July 30, 1957, and 2,843,792,Y issued July 15, 1958, of J. R. Pierce. However, in operation, it is found that the gain and power output that can be realized in such an amplifier are limited for the reasons discussed. As the beam current is increased in an endeavor to eect an increase in gain, the amplifier becomes unstable and oscillations occur. This problem has curtailed the use of bifilar helices in high-gain or high-power forward wave amplifiers.

A study of the characteristics of the bifilar slow wave circuit has shown that substantially all of the microwave energy propagating therealong at a phase velocity within the range which is of interest for interaction with an electron beam will be contained within two distinct modes. ln one of these modes, commonly designated the out-ofphase mode, the currents fiowing on the two helical wires are equal and out of phase at corresponding points in the wires in a plane cutting perpendicularly the bifilar helix axis, and, in the second mode, commonly designated the in-phase mode, the currents flowing ou the two helical wires are equal and in phase at corresponding points in such a plane. Moreover, it has been determined that the in-phase mode is characterized by a high impedance, and is therefore an extremely desirable mode, for operation in a forward wave amplifier. The out-of-phase mode, however, is characterized by strong spatial harmonic components and therefore exhibits a strong tendency to oscillate in a backward wave mode. The presence of the outofphase mode on the bifilar helix, therefore, makes the amplifier highly unstable for high-gain operation or at high power levels since it is then apt to give rise to backward wave oscillations.

A more specific object, therefore, is to increase the power and gain that can be realized in a forward wave amplifier of the traveling wave tube type which utilizes a bifilar helix `as a slow wave interaction circuit.

ln accordance with the present invention, by selectively Vsuppressing the propagation of the out-of-phase mode,

without significantly affecting propagation of the in-phase mode, a stable high-gain amplifier can be realized. To this end, a feature of the present inventionis the cornbination of a bilar helix slow wave interaction circuit and a loading transmission structure closely coupled to the out-of-phase mode thereof, for suppressing propagation of this mode along the bililar helix circuit. An advantage of this arrangement for suppressing the out-ofphase mode is that it is compatible with the use of the biflar helix as a means for electrostatically focusing the electron beam, as explained in greater detail below.

In an illustrative embodiment of the present invention, a traveling wave tube including an electron gun and a target for defining therebetween a path of flow for an electron beam is equipped with a bifilar helix slow wave circuit, The bililar helix, which is a plurality of operating wavelengths long, is positioned to surround the electron beam and extends longitudinally along a portion of the length of the tube for propagating microwave energy in coupling relation with the electron beam. Input and output coupling connectors are provided at opposite ends of the b-ilar helix for coupling wave energy thereto. Additionally, a two-strip transmission line is positioned along the length of the bifilar helix in coupling proximity to the out-of-phase mode thereof, the two strips arranged in parallel on opposite sides of the bifllar helix.

The above and other features and objects of the invention will become apparent by referring to the following description which should be read in conjunction with the accompanying drawings, in which:

Fig. l shows an embodiment of the present invention inV a traveling wave tube amplifier;

Fig. 1A shows a partially c-ut-away view of the twostrip transmission line, with a schematic representation lpl-iiication.

interwound'helices'lS and V16,is positioned to surround `their lproper voltages. static eld is thereby set up along lthe pathof iiow-by Vcoupling helix away from the coaxial connectors.

'establishing a solid electron beam of circular cross section along the length of the envelope between these electrodes. The electron gun and the various electrical connections are shown schematically for purposes of sim- Biilar helix V11, composed of two separate the electron -path' for propagating an electromagnetic wave in ycoupling proximity to the electron ilow and is maintainedat a suitable average potential `for accelerating the electron beam to a velocity substantially` equal to the axial velocity of the in-phase mode of the signal wave traveling along the `biiilar helix. Focusing ofthe electron beam passing axially within the bilar helix may be obtained by maintaining conductor 15 at a suitablevoltage below the average accelerating voltagevand p conductor 16 the'same amount above the average voltage. In Fig. 1 the source of voltage is shown as a bat- Y "tery S'and lead-in conductors 6 and 7 serve to maintain the helical conductors 15 andrl, respectively, 'atYV A spatially alternating electrothe bi-lar helix. This field, which is constant with time butspatiallyalternating, will effectively focus an .electron beam passing within the bilar helix. This ,tech- .nique is disclosed in detail in myUnited States Patent 2,843,776,issued'1uly l5, 1958. Alternatively, magnetic focusing may Vbe employed for focusing the electron beam,as is Vwell known in the art.

l'1" ransducers-in the form of coupled helices, by way-ofV example,lare-provided for intercoupling between the ex- 'ternalcoaxial lines 17 and 18 and bilar helix 11. The

general principles of coupled helix transducers of this kind are-described in United States Patent 2,811,673,

issued October 29, 1957, of R. Kompfner. In operation as a forward wave amplifier coaxial line 17 serves at the input terminal and the wave energy propagating therealong is coupled to bitilar helix 11 via thesingle conductor helix 21. The wave energy is amplified in passing along the helical slow wave circuit in the direction of electron iiow by the now well known-process of energy interchange between the electrons in motion and the electromagnetic field. The amplified wave is then coupled fromfthe helical slow wave circuit 11 to the singlev Vconducto-r helix 22 and passes along coaxial line 18 to utilization means. The coupling helices 21 and 22V are preferably terminated in impedances 23 and 24, Vrespectively, for minimizing reflections at the end of the It is important also to maintain the bilar helix circuit sufficiently lossy to attenuate wave energy reilected back from the output toward the input by any impedance mismatch between the bililar helix 11 and the single conductor helix 22. The mismatch, however, can be minimized by careful-design as explained in the abovementioned Kompfner patent.

As explained in the above-mentioned patent of R. Kompfner, the amount of coupling between input and ontput couplingV helices 21 and 22 and biiilar helix 11 depends on the winding sense of each. By winding the coupling helices in a direction opposite to that of helix 11,l as shown in Fig. l, the amount of coupling therebetween can be maximized. Moreover, each of the coupling helices should have a pitch such tha-t the axial phase velocity of a wave propagating thereal'xng in the absence of the bilar helix would be substantially the same as'the axial phase velocityof the ill-phase mode of teraction circuit introducesproblems which tendlltolimit the operating level of the amplifier. vThe presence of an ont-o-,phase mode, which gives rise to regenerative feedback when the beam current is sufliciently high, results in backward wave oscillations.V `Moreover, excitation of this out-of-phase modeV is independent of the manner in which the input signal is applied to the biiilar helix. Since the electron beam is inherently inhomogeneous,

,itV includes noise'components over essentially the whole frequency Vspectrum which is capable of propagating along the bilar helix. As a result, the'bilar helix nor'- mallyv will have inducedthereon wave `components which 'tendto propagate Vtherealong in the out-of-phase mode and -give V'rise-to oscillation at yhigh levels of beam Acurrent. By suppressing this out-of-phase mode, however,

'inra manner which little affectsV the in-phase mode, stable "amplifier operation with--highvalues of -beam current, with consequent high gains-and power levels-can be obtained. l

^For` an understanding of theoperationof thepresent 0 invention, it lwill be .'helpfulto keep in mind the two f 3 4conditions necessary forclose-coupling between two transmission lines. The iirstcondition is that'the eld pattern of "a wavepassing 'along each of the lines be substantially the'same, and thesecond is that the phase velocity 'of Vthe two lines when not k'affected by any coupling "betweenjthe two Vbe-approximatelyequaL Moreover, it `is importantalso to keep in mind thattheresulting propagation characteristics' of 'two transmission lines when closely coupled are substantially different than the propagating chara'cteristics'ofY either of the lines uncoupled. It is this phenomenon which'is utilized to change suliiciently thepropagating Ycharacteristics for the out-ofphase mode that aregion of infinite attenuation or zero transmission" can be obtained. This will be illustrated hereinafter with reference to Fig. 2.

In accordance with an illustrative embodiment of the present invention a two-conductor loading transmission /line 25, 26, is positioned along a portion ofthe length of the bilar helix of Figfl'.' This' transmission line may be undulated asV shown, in order to secure a desired axial phase velocity therealong. By a proper choice ofthe axial 'phase Vvelocity yofv the loading transmission line, the biflar helix dimensions and vthe'beam accelerating potential, there'is effected a strong coupling between'the loading transmission line and the out-of-phase mode on the biiilar helix. VThis coupling results in the suppression of the out-of-phase mode and stable forward wave amplifier operation.

A sketch of the teld pattern 28 along the two conductor loading'transmission line and the field pattern 29 of'the out-of-phase mode along the bilar helix is shown in Fig. 1A. In this figure the bilar helix 11 is shown schematically positioned within the two conductor line 25, 26.` `A portion of conductor 25 is'cutcaway to show the field pattern of the out-of-phase mo'de along the helix. It can be seen that the eld pattern of the out-of-phase mode on the helix, shown by thebroken lines 29, is substantiallythe same as that of a wave passing along the two. conductor line. It will be .appreciated by a worker in the .art that the eld pattern of the irl-phase mode o'n the helix is substantially different than the iield pattern along the loading transmission line.

The second condition is that thephase velocity of the out-of-phase mode'on the bifilar helix, when not in the presence of the loading transmission line, be approximately equal to' the phase velocity of the transmission line when not in the presence of the helix. It will be convenient to refer to the phase velocity 'of each of these components when unaffected by external coupling as its characteristic phase velocity. A better understanding of the effect of this condition can be obtained by reference to the graph o'f Fig. 2. In this graph the three curves a, b, and c represent the propagating characteristics of the in-phase mode, the out-of-phase mode, and the loading transmission line, respectively plotted against a measure of frequency. It is convenient to plot ,8a against ka where is the propagation constant, a is the helix radius and k is given by the expression (c being the speed of light in free space), and so is a quantityV proportional to frequency. The ratio of the coordinates at any point on 'each of the curves is a measure of the ratio of the velocity of light to the characteristic phase velocity o'f the corresponding component wave for that value of ka. It can be seen that the characteristic phase velocity associated with the in-phase mode is appreciably different from the characteristic phase velocity associated with the loading transmission line, but both being substantially constant with changes in frequency. Because of this and also since they have dierent field patterns, there will be an insignificant amount o'f coupling betwen these modes. It can be noted, however, that the characteristic phase velocity associated with the out-of-phase mode is not constant, but varies with frequency and equals the characteristic phase velocity of the loading transmission line atthe intersection of curves b and c. In the region of intersection the second condition necessary for the close coupling is fulfilled and a strong coupling between the outof-phase mode and the loading transmission line will result. For reasons explained above, in this region of close coupling the propagating characteristics can 11o longer be described by the individual curves b and c which represent the networks uncoupled but are now shown by the broken line curve 30 of Fig. 2. It will be seen that this curve 30 representing the propagating characteristics for the out-of-phase mode along the two lines coupled together, is composed of two segments, one segment o'f the curve for frequencies above and the other for frequencies below the region of equal phase velocity of the loading transmission line and the out-of-phase mode of the bifilar helix. The frequency band between these two broken line segments may be described as a region of zero transmission or infinite attenuation of the out-of-phase mode. This frequency region of no propagation for the out-of-phase mode, which results as a consequence of the presence of the two-strip transmission line, `shall be designated a stop band for the out-of-phase mode.

However, the effective region of suppression for outof-phase mode actually extends outside of the sto'p band, but the attenuation outside of this stop band is considerably less. It has been found, however, that the attenuation outside of the topV band can be increased,

thereby extending the frequency region of high attenuai -tion, by inserting lo'ssy material on the loading transmission line. In Fig. l a cloth 27 of lossy material such as Aquadag is inserted along the loading transmission strips 25 and 26 for extending the frequency range of high attenuation for the out-of-phase mode outside the region of the sto'p band shown in Fig. 2.

The significance of maximizing the range of frequencies over which high attenuation of the out-of-phase mode extends can be explained in the following manner. It is well known by those skilled in the art that in a backward wave oscillator the frequency of oscillatio'n is dependent upon the voltage applied to accelerate the electron beam. When the accelerating voltage is changed the frequency of oscillation will change. However, in

the present device, as long as the theoretical frequency of oscillation falls within the regio'n 'of high attenuation for the out-of-phase mode,` oscillation will be impeded. It follows, therefore, that for a high attenuation reg-ion extending over a very wide frequency range considerable changes mayv be made in the beam accelerating voltage of the amplifier without resulting in backward wave oscillation of the out-of-phase mode. This wide choice in the selection of the accelerating voltage is useful both in design and operationl of forward wave amplifiers.

In the design. of a forward wave amplifier utilizing an interaction circuit which is characterized by high impedance` for both forward and backward wave modes,

made so that the .desired frequency for amplification lies within the stop band for the backward wave mode. However, this restrictionin the choice of design parameters can be circumvented to a large extent by extending the range of high attenuation for the backward wave mode over a wider range of frequency. As explained above, this can be accomplished by inserting lossy material on the loading transmission line. So, after choosing the design parameters to obtain the desired amplifier operating characteristics, if the desired frequency for amplification fallsk outside of the frequencykrange of the stop band, lossy material may bey inserted yalong the loading line in order to extend the range of high attenuation for the backward wave mode, thereby effectively suppressing oscillation.

The extremely wide frequency range over which the present amplifier can operate is indicated by the curves of Fig. 2. In this figure it should be noted that the stop band for the out-of-phase mode, which represents the region of stable amplifier operation even before lossy material is introduced along the loading transmission line, extends to the value of ka equal to 1.5. ka is the ratio of the helix circumference to the free-space wavelength, a quantity directly proportional to frequency. In comparison, it is noted that the usual slow wave circuit such as the single wire helix, in the absence of a loading transmission line which characterizes the present invention, has an operating range which extends only to the value of ka equal to 0.3. This means that for a given helix circumference the present amplifier will have stable operation at a frequency five times higher than usual arnplifiers. Alternately, for a given operating frequency the present amplifier can use a helix whose circumference is five times as large as the circumference of single wire helices in amplifiers proposed heretofore. This obviously will extend the useful range of forward wave amplifiers to bandwidths which have heretofore been unobtainable. Moreover, such large circumference helices will be especially useful at high power operation where large beam currents are important.

Fig. 3 shows an alternative modification of the present invention. Elements of this figure corresponding to elements of Fig. 1 are designated by the same reference numerals. The two conductor transmission line of Fig. 1 is replaced by a single wire helix which, by way of example, is shown wound on a fiat strip of dielectric material. In accordance with the present invention the flat helix 31 is positioned adjacent the tube envelope 14 in coupling proximity to the bifilar helix 11 and serves as a loading transmission line. It can be shown that the electric field pattern associated with the flat helix is substantially the same as that of the out-of-phase mode of the bifilar helix. This fulfills the first of the conditions necessary for strong coupling which are setforth above.

vMoreover, by-adjusting the pitch of theiiat helix so that its characteristic phase velocity is approximately equal to the characteristic phase velocity-of the out-of-phase mode along the'biiilar helix, the second condition'essential Vto strong coupling will be satisfied. By positioninga fiat helix having-such a pitch in coupling proximity to bilar helix 11 the energy`v propagated in the out-of-phase mode along-helix 11 will be suppressed and a stop Vband thereby produced, as explained above with reference to Fig. 2.

Also, as explained with reference to Fig. l, the region of high attenuation vmay be extended outside of theregion of the stop band by making the external loading transmission line lossy. This is done by coating-the dielectric strip 32 with a lossy material 33 such as Aquadag. .1

A cross section. of- Fig..3 -taken throughline 3A-3A is shown in Fig. 3A. This iigure shows-more clearly the Wide dimension of the flat helix 31. This helix is wound on a flat-strip of dielectric material 32 which is coated with a lossy material 33, and preferably positioned close to the biiilar helix 11, whichV is within envelope 14, to insure close coupling with the out-of-phase mode on the biiilar helix.

lt is understood that the specific Yembodiments are tion. Various other arrangements may be devised by one skilled in theart without departing from the -sp'irit and scope of the invention. In particular, the basic principle of the invention, the loading of an interaction circuit which is susceptible to both forward and backward wave modes for suppression ofthe backward Wave mode, may be extended to forms of interaction circuits other than the biiilar helix, such as a single wire helix circuit or a' circuit comprising a pair of coaxial helices of different diameters and wound in opposite senses.

What is claimed is:`

l. in a device which utilizes the interaction between yelectromagnetic wave energy and an electron beam, an

envelope, an electron gun and target spaced apart in said envelope for defining a path of electron flow, and means for propagating electromagnetic wave energy in coupling proximity with saidv electron flow, ysaid means Ncomprising a slow wave circuit capable of propagating both forward and backward wave modes and a transmis- Vsion line positioned in couplingrelation selectively to the backward wave mode of said slow wave circuit for creating in said slow wave circuit a stop-band. or region of no propagation for said backward wave mode.

2. In a device which utilizes theV interaction between envelope, an electron gun and target spaced apart in said envelope fordeining a path of electron iiow, and

Vmeans for propagating electromagnetic wave energy in merely illustrative of the general principlesv ofthe inven- ,electromagnetic wave energy and an electron beam, an

coupling proximity with said electron flow, said Vmeans comprising a slow wave circuit capable of propagating both forward and backward wave modes and a two conductor transmission line extending'along the length of the slow wave circuit'for creating in'said slow wave circuit a stop-band for said backward wave mode, the two conductors being undulated and arranged in parallel on opposite sides ofsaid slow wave circuit.

3. In a device which utilizes the interactionV betweenV for .creating vin the slow wave circuit over a wide `frequencyrange a stop-band for the backward wave mode. 4. In a device which lutilizes the` interaction between electromagnetic wave Aenergy and an electron beam, an

envelope, an electron gun and target spaced apart Vin said envelope for defining a path of electron ow, means for propagating electromagnetic wave energy in coupling proximity with said electron flow, said meanscomprisiugajslow wave vcircuit capable of propagating forward and backward wave modes and wave guiding means including a iiatV single wire helix wound on a at dielectric strip and positioned along the length of said slow wave circuit in coupling relation selectively to the backward wave mode thereof and a layer of lossy material on Vthe surface of said dielectric strip for creating in the slow wave circuit over a wide frequency range a stop-band for the backward wavemode. Y

5. In a device` which utilizes the Yinteraction between `electromagnetic wave energy and an electron beam, anY

envelope, an electron gun and target spaced apart in said envelope for `defining a path of electron iiow, means for propagating electromagnetic wave energy in coupling proximity with said electron Vflow, said means comprising aV bililar slow wave circuit characterized by a tendency to propagate electromagnetic wave energy in both the in-phase and out-of-phase modes and loading means ,coupled selectively to the out-of-phase modes ofrsaid biiilarcircuit for Vcreating in said circuit a stop-band for the out-of-phase mode.

6. In a device which utilizes the vinteraction between electromagnetic wave energy and an electron beam, an

envelope, an electron gun and. target spaced apart in said envelope for defining a path of electron iiow, means for propagating electromagnetic wave energy in coupling proximity with said electron flow, said means comprising a bitilar helix slow wave circuit 'and a two-strip transmission line coupled Yselectively to the out-of-phase mode of said slow wave circuit for creating in said slow wave circuit a stop-band for the out-of-phase mode.

- 7.In aV device which utilizes the interaction between electromagnetic wave energy and an electron beam, an envelope, an electron gun andV target spaced apart iu said envelope for defining a path of electron flow, means for propagating electromagnetic wave energy in coupling proximity with said electron ow, said means comprising a bilar helix slow wave circuit and a single wire helix positioned along the length of said bilar slow wave circuit in coupling relation selectively to the out-- of-phase mode of said circuit for creating in said circuit a stop-band for said out-of-phase mode.

8. In a device which utilizes the interaction between electromagnetic wave kenergy and an electron beam,'an

envelope, an electron Ygun and targetspaced apart inV said envelope for defining a path of electron iiow, means for propagating electromagnetic lwave energy in coupling proximity with saidelectron iiow, said means comprising a bilar helix .slow wave circuit and a two-strip transmission line extending along the length of saidbiiilar helix in coupling-relation selectively to the out-ofphase mode lthereof for creating in said -biiilar helix a stop-band for said out-of-phase mode, the two strips arranged inparallel on opposite sides of said biiilar helix anda lossy material positioned contiguous to and along said .two-strip line. d

9. In a device which utilizes the interaction between electromagnetic waveenergy and anelectron beam, an envelope, Aan electron gun and target spaced apart in said envelope for defining a path of electron iiow, means for propagating electromagnetic wave energy in coupling proximity with said electronV flow, said means comprising a biiilar helix positioned to surround the path of electron ow, and wave guiding means coupled selectively to the out-of-phase modeV of said biiler 'helix for creating in the biflar Yhelix a stop-band for said out-ofphase mode; and means including a voltage source for maintaining the two interwound conductors of the bilar helix at different potentials for establishing a spatially alternating electrostatic focusing eld along the path of electron ow.

10. In combination, an evacuated envelope, two electrodes spaced apart in said evacuated envelope for forming a path of electron flow, and means for propagating electromagnetic wave energy in coupling relation with said electron ow, said means comprising a slow wave circuit having a tendency to propagate energy in the forward and backward wave modes and a loading transmission line adjacent said circuit in coupling relation selectively to the backward wave mode of said circuit for creating in said circuit a stop-band for the backward wave mode.

11. A traveling wave tube comprising an electron gun for forming an electron beam, a biflar helix for propagating electromagnetic wave energy in coupling proximity to said beam, and a two conductor transmission line positioned adjacent the bilar helix for creating in the bilar helix a stop-band for the out-of-phase mode of said biiilar helix.

12. In a traveling wave tube comprising an electron gun and a target spaced apart in said tube for forming an electron beam, a bilar helix positioned to surround said electron beam, means including a voltage source for maintaining the two conductors of said biiilar helix at different potentials for focusing said electron beam, and a loading transmission line positioned along the bifilar helix and coupled selectively to the out-of-phase mode of said bilar helix for creating in the biilar helix a stop-band for said out-of-phase mode.

13. In a traveling wave tube comprising an electron gun and a target spaced apart in said tube for forming an electron beam, a bilar helix positioned to surround said electron beam, means including a voltage source for maintaining the two conductors of said biiilar helix at different potentials for focusing said electron beam, and a at single-wire helix positioned along the length of the bilar vhelix for creating a stop-band for the outof-phase mode of said bilar helix, the characteristic phase velocity of the single-wire helix being approximately equal to the characteristic phase velocity of the out-of-phase mode in the bifilar helix.

14. In a traveling wave tube comprising an electron gun and target spaced apart in said tube for forming an electron beam, a bilar helix positioned to surround said electron beam,- means including a voltage source for maintaining the two conductors of said bilar helix at diiferent potentials for focusing said electron beam, and a twoconductor transmission line positioned along the length of the bilar helix for creating in the bilar helix a stopband for its backward wave mode, the two conductors being undulated and arranged in parallel on opposite sides of said biiilar helix.

l5. An amplier which utilizes the interaction between electromagnetic wave energy and an electron beam comprising means including an electron source and target for dening a path of electron ow, a biiilar helix slow wave circuit positioned along the path of ow for propagating electromagnetic energy in coupling relation with said electron flow, an input connector positioned in coupling proximity to the bilar helix at the source end of the electron path, an output connector positioned in coupling proximity to the biilar helix at the target end of the electron path, and a two-strip transmission line positioned along the length of the bitilar helix in coupling proximity to the out-of-phase mode thereof for creating in said bilar helix a stop-band for said out-of-phase mode, the two strips arranged in parallel on opposite sides of the bifilar helix.

16. A device which utilizes the interaction between electromagnetic wave energy and an electron beam, comprising means forming an electron beam, and circuit means for propagating microwave energy in coupling proximity with said electron beam, said circuit means comprising a slow wave circuit characterized by the ability to propagate both forward and backward wave modes and a loading transmission line adjacent said slow wave circuit in coupling relation selectively to the backward wave mode thereof, for creating in said slow wave circuit a stop-band for said backward wave mode the characteristic phase velocity and field pattern along the loading transmission line being approximately the same as the characteristic phase velocity and field pattern of the backward wave mode along the slow wave circuit but different from that of the forward wave mode along said slow wave circuit.

17. A device which utilizes the interaction between electromagnetic wave energy and an electron beam comprising an evacuated envelope, two electrodes spaced apart in said envelope for dening a path of electron How and circuit means for propagating high frequency energy in coupling relation with said electron ilow, said circuit means comprising a bilar helix slow wave circuit and a loading transmission line adjacent to saidv biilar helix slow wave circuit for creating in said slow wave circuit a stop-band for its backward wave mode, the characteristic phase velocity and eld pattern along the transmission line being approximately the same as the characteristic phase velocity and field pattern of the backward wave mode along said biillar slow wave circuit.

18. In a traveling wave tube, an electron source providing an electron beam, a wave transmission circuit, a plurality of operating wavelengths long positioned in field coupling relation with the beam, said circuit having a tendency to propagate in a plurality of modes, an input connector coupled to the electron source end of the circuit, an output connector coupled to the opposite end of the circuit, and means for creating in said circuit a stopband for the undesirable propagating modes in the circuit comprising a transmission line extending adjacent to the circuit in coupling relation with said undesirable propagating modes.

References Cited in the le of this patent UNITED STATES PATENTS 2,512,468 Percival June 20, 1950 2,654,047 Clavier, et al Sept. 29, 1953 2,725,499 Field Nov. 29, 1955 2,751,561 King June 19, 1956 2,773,213 Dodds Dec. 4, 1956 2,809,321 Johnson et al Oct. 8, 1957 2,811,673 Kompfner Oct. 29, 1957 2,846,613 Pierce Aug. 5, 1958 FOREIGN PATENTS 993,156 IFrance July 18, 1951 1,053,556 France Sept. 30, 1953 

